1. Field of the Invention
The present invention relates to linearization of a power amplifier used in a cellular phone and base station, for example, as well as to a low-distortion power amplifier and the like.
2. Description of the Related Art
FIG. 17 shows a conventional predistortion circuit. As shown in FIG. 17, a signal that is input to an input terminal 1701 is divided to two routes by a divider circuit 1703. In the first route, the signal is input to a combiner 1707 via a delay circuit 1704. On the other hand, in the second route, a distortion is generated by a distortion generation circuit 1705 and the signal is input to the combiner 1707 via a vector adjustment circuit 1706. The two signals are combined with each other by the combiner 1707 and a resulting signal is output from an output terminal 1702 and supplied to a power amplifier (not shown). The predistortion circuit generates a signal for suppressing a distortion of the downstream power amplifier by varying the amplitude and phase of a distortion with the vector adjustment circuit 1706, whereby a distortion at the output of the power amplifier is suppressed. The delay time of the delay circuit 1704 is so set as to equalize the delay times of the first and second routes.
However, in particular, where a transmission signal has a wide bandwidth and the power amplifier performs a class-AB operation, the amplitude and phase characteristics of a intermodulation distortion generated by the amplifier lose balance and hence the effect of linearization(for example, suppressing distortion) degrades.
A more detailed description will be made below. It is assumed that the power amplifier is matched in a wide band and that the gain and the pass phase have no deviations in a transmission band. When two signals having different frequencies and the same amplitude are input, the output voltage VO is given byVO=AO(cos ω1t+cos ω2t)+BOL cos [(2ω1−ω2)t+φ3L]+BOU cos [(2ω2−ω1)t+φ3U]  [Equation 1]where ω1 and ω2 are the angular frequencies of the input signals, AO is the amplitude component of voltages having angular frequencies ω1 and ω2 (among the output voltages), BOL and BOU are the amplitude components of third-order intermodulation distortion voltages occurring on the low-frequency side and the high-frequency side, and φ3L and φ3U are the phase components of the third-order intermodulation distortion voltages occurring on the low-frequency side and the high-frequency side.
In this case, both intermodulation distortions occurring on the low-frequency side and the high-frequency side can be suppressed for by generating a voltage VI given by the following equation with the lonearizer (for example, predistortion circuit) and inputting it to the power amplifier:VI=AI(cos ω1t+cos ω2t)−BIL cos [(2ω1−ω2)t+φ3L]−BIU cos [(2ω2−ω1)t+φ3U]  [Equation 2]where AI is the amplitude component of voltages having angular frequencies ω1 and ω2 and BIL and BIU are the amplitude components of third-order intermodulation distortion voltages occurring on the low-frequency side and the high-frequency side in the predistortion circuit. There are relationships AO=AI·G, BOL=BIL·G, and BOU=BIU·G, where G is the voltage gain of the power amplifier.
According to the conventional technique, the amplitudes and the phases of third-order intermodulation distortion occurring in the predistortion circuit cannot be controlled independently on the low-frequency side and the high-frequency side. That is, BIL, BIU, φ3L, and φ3U cannot be controlled independently.
No particular problems occur with the above conventional linealizer if the amplitude components of third-order intermodulation distortion voltages on the low-frequency side and the high-frequency side are the same and their phase components are also the same.
However, if the amplitude components and/or the phase components of third-order intermodulation distortion voltages on the low-frequency side and the high-frequency side are different from each other as in the case that a transmission signal has a wide bandwidth and the power amplifier performs a class-AB operation, a problem arises that a sufficient linearization(for example, sufficient suppressing distortion) cannot be obtained.